Examples of a method, receiver system, portable secure device and computer program are known e.g. from ‘Functional Model of a conditional access system’, EBU Technical Review 266, Winter 1995, p. 64-77. This publication discloses a functional model of a conditional access reference system. A common interface is used between an Integrated Receiver Decoder and a CA system. The CA system is contained in a low-priced proprietary module which communicates with the IRD via a common interface. The IRD receives a scrambled MPEG-2 Transport Stream. Entitlement Control Messages (ECMs) are used to recover the descrambling control word in the decoder. Data filters filter out the ECMs, which are sent to a security processor in the CA system, which recovers the descrambling control word and returns it to a descrambler. Entitlement Management Messages authorise a viewer to descramble a service. At least the security processor is usually implemented as a smart card.
A problem of the known system is that the security processor returns control words in response to each ECM for which an appropriate EMM has been sent. This opens up the opportunity of hook piracy. Hook piracy involves providing ECMs associated with several different program transport streams to one conditional access sub-system, e.g. a smart card, and intercepting the returned control words, which are then distributed to other receivers than the one used by the authorised subscriber. Thus, one subscription is ‘shared’ by many users.